Both manufacturers and the general public have placed an emphasis in recent years on recycling materials. Recycling materials is desirable from the standpoint of reducing waste that must be landfilled, burned, or otherwise discarded. It has become desirable for manufacturers to include recycled plastic or recycled rubber content in the manufacture of articles. In some instances, recycled materials may also offer a cost advantage over virgin materials.
Recycling of thermoset polymeric materials, however, has faced serious difficulties in the preparation of materials for recycling and in maintaining desired properties for the articles that incorporate recycled material. One avenue for recycling crosslinked rubbers has been the use of ground crosslinked rubber as a filler in new thermoset polymeric materials. Ground crosslinked rubber may be produced in mesh sizes of from 10 to over 300, with a typical range being about 40-80 mesh. The ground crosslinked rubber has commonly been recycled by adding it to uncrosslinked rubber and then vulcanizing the blend rubber. In general, it has been reported in literature that blends cannot include more than about 10% recycled rubber without suffering a significant decrease in properties, including surface roughness. Mixtures including up to 30% ground crosslinked rubber in new rubber have been reported to have somewhat lower tensile strength as compared to virgin rubber.
One solution to the problem of how to incorporate larger amounts of recycled rubber without a decrease in properties is to chemically treat the recycled rubber. Stosky reported in "Innovation in the Development and Use of Recycled Rubber," Paper No. 42, Rubber Division, American Chemical Society (May 6-9, 1997) that chemically treated recycled rubber could be incorporated in amounts of up to 75% in a blend with virgin rubber, resulting in a material having equivalent or even superior performance as compared to virgin rubber. The chemical treatment appears to involve a surface treatment that allows for crosslinks between the ground crosslinked rubber and the virgin thermosetting material. Another method of recycling rubber involves de-vulcanizing the rubber by physical or chemical means. De-vulcanized rubber may be mixed with virgin material, re-molded, and then re-vulcanized along with the new material to produced a crosslinked blend. A process of treating ground crosslinked rubber with a caustic gas to allow bonding with other polymers has also been reported.
These methods, however, involve added reactions and manufacturing steps that make recycling of materials more expensive. Moreover, these methods are directed to thermoset materials only. Thus, the need remains for a relatively simple and effective means of recycling cured rubber scrap into new articles, particularly articles formed from thermoplastic compositions.
The present invention provides a method of recycling a cured EPDM terpolymer or EPR copolymer rubber, or mixtures including these rubbers, by blending the recycle rubber into a thermoplastic polyolefin resin to form a blend composition. The blend compositions comprise ground crosslinked rubber having a particle size of about 80 mesh or smaller, preferably of about 120 mesh or smaller. (The mesh size is inversely proportional to the particle size.) The compositions prepared according to the methods of the invention may include a surprisingly high level of recycle rubber without adverse effect on physical or aesthetic properties. Thus, the blend thermoplastic materials produced may comprise up to about 70% by weight of the ground crosslinked rubber, based upon the total weight of the blend material. The thermoplastic blend materials of the invention also comprise at least one thermoplastic polyolefin resin and a compatibilizer.